Periodontal diseases are among the most prevalent infections affecting otherwise healthy adults in the US. Among the few human genetic traits shown to be associated with periodontitis are polymorphisms in calprotectin (S100A8/S100A9), an innate antimicrobial protein (AMP) complex. We have pioneered the study of S100A8/A9 as a key innate immune effector molecule in epithelial cell autonomous immunity. In periodontal tissues, calprotectin features prominently, localizing in the cytoplasm of mucosal epithelial cells and polymorphonuclear leukocytes (PMNs). Released from lysed cells, citrullinated histone H3, extracellular DNA and S100A8/A9 complex to form neutrophil extracellular traps (NETs) in periodontal lesions or ?pockets.? Seemingly protective against infection in abscesses and perhaps in the periodontal pocket, S100A8/A9 released into other tissue compartments has also been suggested to be a danger-associated molecular pattern (DAMP; Alarmin), potentially inducing a tissue- destructive proinflammatory response. Since S100A8/A9 in the periodontal pocket resides within the cytoplasm of epithelial cells and PMNs or is released from lysed PMNs and desquamated epithelial cells to complex in NETs, we hypothesize that S100A8/A9 contributes to resistance to experimental periodontitis and alveolar bone loss in C57BL/6J mice. To test our hypothesis, we will compare ligature-induced experimental periodontitis when S100A8/A9 is present or absent in three major compartments: mucosal epithelial cells, PMNs and NETs. Specifically, we will (1) show whether S100A8/A9 impacts the resident oral mucosal microflora and (2) demonstrate that S100A8/A9 protects against signs of periodontitis. These experiments will characterize the role of S100A8/A9 produced by gingival epithelial cells and PMNs in modulating the oral flora and in resistance to experimental periodontitis. The findings will elucidate the reported genetic association between S100A8/A9 and human periodontitis. Since we have reported that invading bacteria and IL-1? cause upregulation of S100A8/A9 through activation of C/EBP?, future studies will determine the impact of this regulatory mechanism on the expression of S100A8/A9 in vivo and barrier protection against periodontal infections. Furthermore, the data from these experiments may also suggest that augmenting S100A8/A9 may be a novel therapeutic strategy for the prevention and treatment of human periodontitis.